U.S. Pat. No. 5,611,670 shows a gas turbine moving blade made of a nickel based superalloy. Superalloys are nickel- or cobalt-based alloys, typically comprising chromium, titanium, tantalum, aluminum, tungsten and other elements, with excellent high temperature resistance, thereby maintaining high strength properties. Accordingly, superalloys are widely used in high temperature applications where additionally high mechanical strength is required. A typical application is the casting of airfoils for gas turbines, jet engines as well as stationary gas turbines, e.g. for industrial applications like power generation. Further improvements in mechanical strength is achieved by casting the superalloy as a columnar or as a single crystal. A textured article has no or very few grain boundaries.
European Patent Application EP 1 038 982 A1 describes a process for manufacturing single crystal superalloy articles. After casting the article is subjected to a heat treatment in order to further improve the mechanical strength. The heat treatment is a high temperature solution heat treatment which homogenizes the microstructure of the alloy itself formed by different crystal phases. However, this heat treatment may lead to a grain recrystallization process, initiated by dislocations in the crystal structure. This grain recrystallization destroys locally the single crystal structure which may lead to a dramatic decrease in the mechanical strength of the article. Accordingly, grain recrystallization is a cause for rejection of single crystal castings if present beyond a preset maximum for recrystallized grains and can result in quite low yields of acceptable heat-treated single crystal castings. By heat treating in a carburizing atmosphere, carbon is introduced into the casting and forms carbides therein that reduce or localize grain recrystallization.
European Patent EP 0 525 545 B1 describes the refurbishment of corroded superalloy articles. In particular gas turbine airfoils are subjected to corrosion by hot gases. Typically, a corrosion protective coating is provided on the body made from the superalloy. Widely used are coatings of the type MCrAlY, where M is iron, cobalt and/or nickel, and Y stands for yttrium or another rare earth element or another element such as lanthanum. This type of coating is usually provided by a plasma spray process. However, despite a corrosion protective coating, the airfoils are still under corrosion and erosion attack which leads to the need for servicing after a certain time period. Corrosion results from contaminants in the fuel and/or air; furthermore, oxidation may also occur at high temperatures. Depending on the conditions of operation, an oxide layer of varying thickness may form on the surface of the airfoil. Also, and very significantly, sulfur can penetrate into the base material to form sulfides. Also, internal oxides and nitrides may form within the metal near the surface. Instead of completely exchanging airfoils, it is often a cost saving option to refurbish the airfoils, i.e. providing a new protective coating. This requires complete removal of the old coating, which is realized by applying mechanical stripping as well as chemical treatment, e.g. with acid. After removal of a substantial part of the old coating, the surface is aluminized. Subsequently, the aluminide layer is removed, thereby also removing oxidized and corroded regions at the surface.
The U.S. Pat. No. 5,413,648 discloses a directionally solidified article with a plastic deformation damage at the surface, which is prone to recrystallization. This problem is overcome by removing a part of the deformed surface region.
The EP 1 036 850 A1 discloses a single crystal having a surface coating for preventing recrystallization fracture by reinforcing the grain boundaries. After a heat treatment the texture of the article at the surface shows no single crystal structure anymore, because the surface has grain boundaries, which are reinforced by grain boundary strengthening elements like Zr, Hf, B or C.
The U.S. Pat. No. 6,271,668 shows the need of a coating during one step of refurbishment of gas turbine components, which is applied on the surface of the component, which is heat treated with this coating by which a surface region of the article is aluminized. This heat treatment is performed at low temperatures in order to avoid detrimental diffusion of atoms from corrosion products.
The coating is removed together with the corroded layers before further heat treatments are performed.
U.S. Pat. No. 6,024,792 has disclosed a process for build-up welding. During the build-up welding, a laser beam or electron beam is used to melt powder and allow directional solidification thereof, so that the region which has undergone build-up welding has a directionally solidified structure like the underlying substrate.